# HG changeset patch
# User Jordi Gutiérrez Hermoso <jordigh@octave.org>
# Date 1513485505 18000
# Node ID 8d1edc9efcb4b426a2c6b3a248ba6788161cd700
# Parent  6ee82d2c10fadb382d1cf8c7b944d39818bd94c0
day 16

diff --git a/2017/day16.d b/2017/day16.d
new file mode 100644
--- /dev/null
+++ b/2017/day16.d
@@ -0,0 +1,49 @@
+import std.stdio;
+import std.algorithm: swap, bringToFront, map, find, canFind;
+import std.string: split, indexOf;
+import std.conv: to;
+import std.range: iota, join, zip;
+import std.array: array;
+
+auto parseOps(string[] ops, string progsin) {
+  // ubyte[] not char[] because https://issues.dlang.org/show_bug.cgi?id=16959
+  auto progs = cast(ubyte[]) progsin.dup;
+  foreach(op; ops) {
+    switch(op[0]) {
+    case 's':
+      auto rot = to!int(op[1..$]);
+      bringToFront(progs[rot..$], progs[0..rot]);
+      break;
+    case 'x':
+      auto idx = op[1..$].split("/").map!(to!int);
+      swap(progs[idx[0]], progs[idx[1]]);
+      break;
+    case 'p':
+      auto AB = op[1..$].split("/");
+      swap(find(progs, AB[0])[0], find(progs, AB[1])[0]);
+      break;
+    default:
+    }
+  }
+  return cast(string) progs.idup;
+}
+
+auto iteratePerm(string[] ops, string progs) {
+  string[] seenprogs = [progs];
+  string result = progs;
+  while(true) {
+    result = ops.parseOps(result);
+    if(seenprogs.canFind(result)) {
+      break;
+    }
+    seenprogs ~= result;
+  }
+  return seenprogs[1_000_000_000 % seenprogs.length];
+}
+
+void main(string[] args){
+  auto ops = File(args[1]).byLineCopy.front.split(",");
+  auto progs = "abcdefghijklmnop";
+  writeln(ops.parseOps(progs));
+  writeln(ops.iteratePerm(progs));
+}